The production of most powder metallurgy parts involves two major steps: compaction and sintering. The compacted, or green, parts are fragile unless sintered.
Sintering is the process of heating a green compact, usually in a protective atmosphere, to a temperature below its melting point to cause its particles to bond together. The mechanism is based upon the diffusion of metal atoms between the individual powder particles.
The process typically comprises passing the green powder metallurgy compacts through a sintering furnace comprised of a pre-heat section, a high-temperature (hot zone) section and a cooling section which sections are supplied with a protective atmosphere. Conventional sintering temperatures in the hot zone commonly range from about 2,000.degree. to 2,100.degree. F. (1,093.degree. to 1,149.degree. C.) due to the limitations of the materials used in common sintering furnaces.
Probably the most widely used protective atmosphere to date is endothermic gas which comprises about 40% nitrogen, about 20% carbon monoxide, and about 40% hydrogen. Endothermic gas is generated by the controlled partial oxidation of natural gas or other hydrocarbon sources. Sintering under high quality endothermic gas at a temperature of about 2,050.degree. F. (1,121.degree. C.) provides an acceptable carbon potential.
Exothermic gas which is generated from burning about 6 parts of air with 1 part of natural gas and subsequently removing carbon dioxide and moisture is also used as a protective atmosphere in sintering processes. This atmosphere comprises about 75% nitrogen, 11% carbon monoxide and about 13% hydrogen. Exothermic gas is usually used as a protective atmosphere during sintering of powder metallurgy parts only when carbon potential is not important.
Dissociated ammonia which comprises 25% nitrogen and 75% hydrogen is also used as a protective sintering atmosphere. For sintering carbon containing compacts, however, dissociated ammonia suffers from a drawback in that it contains no hydrocarbon constituents to counteract decarburization.
More recently, the trend has been towards the use of protective atmospheres comprising predominantly nitrogen to which controlled amounts of other gaseous components such as carbon monoxide, hydrogen, hydrocarbons and even water have been added. U.S. Pat. Nos. 4,016,011; 4,106,931; and 4,139,375 are representative.
U.S. Pat. No. 4,016,011 discloses a method for the heat treatment of a high-alloy steel article in an atmosphere comprising 0.5 to 1.5% carbon monoxide, 0.5 to 2.5% hydrogen, and a small amount of active carbon with the remainder being nitrogen. The atmosphere is generated by the thermal cracking of a liquid organic compound such as isopropanol or methyl acetate. Heat treating temperatures of 1,000.degree. to 1,200.degree. C. and up are mentioned.
U.S. Pat. No. 4,106,931 describes a method for sintering carbon steel powder metallurgy parts having a density of less than 90% theoretical density and 0.3 to 1.3% carbon in the form of graphite. The part is heated in a hot zone to a temperature of at least 2,000.degree. F. in a controlled atmosphere of at least 90% nitrogen, up to 9.75% hydrogen and carbon monoxide, with the carbon monoxide being less than 5.0%; 0.25 to 2% methane or equivalent hydrocarbon and a dew point of less than -60.degree. F.
U.S. Pat. No. 4,139,375 discloses sintering powder metal parts in a furnace having 2 successive zones, one of which is an upstream zone maintained at a temperature in the range of about 800.degree. to 2,200.degree. F. A gaseous mixture consisting essentially of methanol and nitrogen is introduced into the upstream zone at a point where a temperature of at least about 1,500.degree. F. is maintained. The methanol and nitrogen are in a ratio sufficient to provide an atmosphere comprising about 1 to 20% carbon monoxide, about 1 to 40% hydrogen and the balance nitrogen. It is suggested that amounts of an enriching gas such as methane or other hydrocarbons be introduced into the atmosphere in a range from about 1 to 10%.
A goal of any sintering process is the minimization of decarburization in the core of the metallurgical part along with control of surface carbon for improved strength, size control and aesthetic features such as surface luster.
However, it is nevertheless customary and accepted to sustain a maximum of about 0.15 to 0.20% carbon loss with respect to parts formed of atomized or sponge-type powders. Accordingly, if carbon is present in the green compact at a level of 0.9% as graphite, an acceptable part after the sintering process would have a core that is at least 0.7% carbon. The function of the protective atmosphere is to prevent further carbon loss.
A further goal in the sintering process is to prevent excess carburization of the compacts. Excessive carbon potential of the atmosphere can result in a degradation of physical properties caused by iron carbides and also in soot deposition on the compacts and in the furnace.
Representative of literature references extolling high temperature sintering is J. R. Merhar, "The Application of High Temperature Sintering in the Production of P/M Components," Hoeganaes P/M Technical Conference, Philadelphia, Pa. 1978 which indicates that the temperature at which parts are sintered may have the greatest influence on mechanical properties, and that the sintering atmosphere selected may also have a subtle influence on properties. Increasing temperatures above the conventional 2,050.degree. F. can improve mechanical properties such as impact strength and the ductility of stainless steel powder compacts.
However, problems including the above-described decarburization and surface carbon loss of the metallurgy part, which are encountered in sintering processes at conventional temperatures of about 2,000.degree. to 2,100.degree. F. (1,093.degree. to 1,149.degree. C.), are substantially magnified if high temperatures above 2,200.degree. F. (1,204.degree. C.) are employed. Sintering at such high temperatures enhances the decarburizing rate of hydrogen, carbon dioxide, oxygen and water found in conventional furnace atmospheres. The result is an excessive carbon loss from the powder metallurgy compact. Conventional furnace atmospheres which contain hydrocarbons can cause excessive carbon pick-up, or recarburization, due to the high carburizing rates at these higher temperatures.
Atmosphere control and purity are extremely critical at temperatures greater than 2,200.degree. F. (1,204.degree. C.). An endothermic gas atmosphere may not provide sufficient carbon potential. The resulting decarburization from the excessive carbon dioxide and water in endothermic gas can render it impractical for high temperature sintering.
In sum, the difficulties encountered in controlling recarburization or decarburization when using prior art protective atmospheres at the conventional sintering temperatures became even more pronounced at the higher sintering temperatures of greater than about 2,200.degree. F.
S. Mocarski et al., "High Temperature Sintering of Ferrous Powder Metal in Nitrogen Base Atmospheres," Metal Progress, December 1979 disclose a nitrogen base atmosphere comprising 96 parts nitrogen and 4 parts hydrogen with a small addition of carbon monoxide or methane.